1. Field of the Invention
The invention relates to a solar module, comprising a solar cell and a converter, and to a method for its operation.
2. Description of the Related Art
Solar cells are commonly used to generate regenerative energy. To prepare the DC voltage generated by the solar cells for use, for example in a power supply network, they are combined with a converter, generally an inverter, to form a solar module. The converter converts the DC voltage received by the converter from the solar cells into an AC voltage which is in turn supplied to the power supply network. The solar cell generally mentioned in the singular here in practice often comprises a plurality of individual solar cells connected in series. In a known, simple embodiment, the converter has at least two transistor half bridges. Each half bridge contains then at least two transistors with at least two freewheeling diodes. Further semiconductors can be located between the solar cell and the converter. As a rule, therefore, the DC voltage generated by the solar cell is fed into the AC network by means of an inverter. To this end, a pulse pattern is generated with which the transistors of the inverter are switched.
Non-illuminated or weakly illuminated solar cells supply no DC voltage or too low a DC voltage to have this processed further by the converter. The converter is therefore initially inoperative. When the solar cell begins to be illuminated, its no-load voltage rises quickly to a value that is higher than in normal operation. The no-load voltage is present when no or only a very low load current flows through the solar cell. As compared with an output voltage when the solar cell is loaded with a load current, this value is very high, frequently about twice as high. This is present on the internal resistance of the solar cell.
In particular at cold ambient temperatures, which means in winter and at sunrise, the solar cell has a maximum-no-load voltage. Consequently, the DC voltage present on the inverter in the form of its intermediate circuit voltage also reaches a maximum, since the inverter is not yet in operation and therefore no noticeable current flows through the solar cell. During operation of the inverter, i.e., when a load voltage is drawn from the solar cell, the DC voltage drops substantially, for example from a peak voltage of 900 V to a regular (operating) voltage of 500 V. The voltage output by the solar cell therefore depends, inter alia, on the solar irradiation, the temperature and the current flow through the cell.
Thus, it is always possible to think of a starting or transition operation in which the output voltage from the solar cell is above the permitted input voltage of the converter.
It is known to use an inverter for the solar cell which is designed only for the operating voltage of the solar cell, i.e., for 500 V in the example. In the starting case outlined above, however, the permissible operating voltage of the inverter is exceeded considerably by the no-load voltage of the solar cell of 900 V. Direct starting is therefore not permissible under these circumstances because of the maximum operating voltage of the solar cell being exceeded by the no-load voltage of the solar cell.
It is known, during start-up operation of the solar cell, to assign to the input of the inverter a device for the temporary reduction of the intermediate circuit voltage, a chopper, for example with a load resistance. Before the inverter is connected, the solar cell is loaded by the chopper. A load current flows in the solar cell. The current flowing through the chopper in this case loads the solar cell, so that its voltage value and thus the intermediate circuit voltage to be fed to the converter, falls to a value which is in the permitted voltage range of the inverter. The input voltage, which means the intermediate circuit voltage, at the inverter is therefore reduced to the maximum permissible intermediate circuit voltage. The inverter can then be started up, the chopper becomes inactive, the inverter can start up and feed power into the network. During start-up operation of the solar cell, a chopper is therefore needed in such a configuration.
Alternatively, it is known to use fewer solar cells in a solar module in order to lower its maximum voltage, or to dimension the converter for the maximum available no-load voltage of the solar cell. The voltage on the converter is determined by the number of solar cells connected in series. In this case, the highest possible voltage is aimed for, since the most power can then be transferred with an inverter. The maximum no-load voltage, which means the maximum voltage fed to the converter, is always below the permissible converter voltage or intermediate circuit voltage even in start-up or transition operation. However, since the transition operation lasts only a few seconds as compared with many hours of normal operation, during normal operation the converter then operates considerably below its design limits for nearly all of its operation. The semiconductors in the converter will be loaded fully in terms of voltage only for a few seconds; in the rest of the time the converter is over-dimensioned. As soon as the converter works towards the network, the voltage decreases considerably, as described above.
This over-dimensioning of the circuitry adds considerably to its manufacturing cost.